California Code, Civil Code - CIV § 988

(a) For the purpose of this section:

(b) Whenever an exclusive or nonexclusive conveyance of any right to reproduce, prepare derivative works based on, distribute copies of, publicly perform, or publicly display a work of art is made by or on behalf of the artist who created it or the owner at the time of the conveyance, ownership of the physical work of art shall remain with and be reserved to the artist or owner, as the case may be, unless such right of ownership is expressly transferred by an instrument, note, memorandum, or other writing, signed by the artist, the owner, or their duly authorized agent.

(c) Whenever an exclusive or nonexclusive conveyance of any right to reproduce, prepare derivative works based on, distribute copies of, publicly perform, or publicly display a work of art is made by or on behalf of the artist who created it or the owner at the time of the conveyance, any ambiguity with respect to the nature or extent of the rights conveyed shall be resolved in favor of the reservation of rights by the artist or owner, unless in any given case the federal copyright law provides to the contrary.